[Elecraft] K3 PA temperature reading jumps / hot smell

[Felix Hormuth]

Dear all,

my K3 (S/N 8862) is developing a strange behaviour that is beginning
to worry me a bit... Current firmware, assembled as kit, power
calibration just repeated to be sure.

First -- and this is not really new -- from time to time it produces
a smell that is best described as 'hot', like in hot resistor, hot
transistor and similar. I've never managed to track down the source,
though it seems to be related to output power. Sometimes it appeared
after just 1 minute of operation (the maximum I do is 100W RTTY with
short QSOs and never more than 50% duty cycle) and was then gone
for the rest of the day, sometimes it started a bit later, and sometimes
it never appeared. The PA temperature was always fine in these cases,
below 50 degC max. Now this smell seems to be increasing and I'm
worried some real damage might develop.
The second new issue is that during TX the PA temperature reading
increases quite fast and then when switching back to RX *immediately*
drops back by several deg C.
Example:

- start at PA temp = 33C
- transmit 90W RTTY for 30s on 14MHz into dummy load
--> temperature increases to 43 deg C
- back to RX: temperature drops down instantly to 35 deg C

Now the funny thing is: if I do that on 10MHz, it behaves
completely normal. Temperature increase is just 2C, no drop at the end.
That's how it was on 20m in the past as well.
On 15m somewhere in between (+4, and then drop by 2), then on 12m and
higher again back to normal.

Anyone with similar observations? Any ideas what to check or test or do?

Man thanks for any help,
vy 73, Felix DL2FHO